The vorticity in any gas or liquid having a relative motion is one of the more fundamental properties of that motion. It is a well defined quantity in fluid mechanics and is related to the angular velocity of the fluid at every point, and hence ranks in importance to the velocity itself. Since vorticity is always present whenever there is relative motion between a fluid and a solid surface, it determines how much energy must be expended in order to maintain the relative motion. In spite of its importance, few vorticity measurements have been made because many conventional types of vorticity probes are either fragile or cumbersome. One type of vorticity probe, commonly known as a Kovansznay vorticity meter, utilizes precisely heated wires of approximately 0.0001 inches in diameter. The wires are oriented in a predetermined geometrical configuration and measure changes in the velocities of the flowing fluid by sensing temperature changes within the wires. This type of probe is extremely complicated and fragile. A second type of vorticity meter, called a vane-type probe, is provided with vanes which rotate at an angular rate proportional to the angular velocity of the flowing fluid. Because of its large size and problems associated with friction and complicated output instrumentation, its use has been confined primarily to applications requiring only visual indications of vorticity.
The vorticity measurement probe of the present invention eliminates the above-described problems. It is rugged, of small size and provides an output not requiring complicated output instrumentation or analysis techniques. The probe comprises an elongated stem member having one end fixedly mounted to a holding structure that orients the longitudinal axis of the stem member at a predetermined angle with respect to the desired flow of fluid to be measured. In one embodiment, the stem member is formed with a rotation-preventing indentation at one end which is slidably received by the holding structure. The other end of the stem member incorporates means for imparting a torque to the stem member proportional to the vorticity of the flowing fluid. The torque imparting means can be defined by any surface configuration sufficient to impart the desired torque as exemplified by fixedly mounted fins or by a sphere having a roughened outer surface. Finally, the invention comprises means for measuring the torque developed by the torque imparting means. In one embodiment the measurement means consists of two longitudinally extending strain gauges spirally wound on the stem member so that their principal axes are at 45 degrees with respect to the longitudinal axis and are aligned with the principal axis of the stem member. The strain gauges are oppositely wound with respect to each other thereby forming a 90 degree angle at their intersection.
Another embodiment of the measurement means consists of four additional longitudinally extending strain gauges mounted at 90 degree intervals around the circumference of the stem member. These gauges are used to determine the component of the fluid velocity flow which is normal to that portion of the flow parallel to the longitudinal axis of the stem member. They can be used in conjunction with the spirally wound strain gauges described above.
The vorticity measurement device of the present invention does not require any moving parts. By appropriately choosing the stem material and utilizing strain gauges, or any of several other torque measuring devices, a signal proportional to vorticity can be obtained for a wide range of vorticity conditions. The lack of moving parts and the need for only elementary electronics provides a simple, rugged and highly reliable device heretofore unavailable in the art.